Filtration systems are known which are comprised of filter elements that are a stack of serrated annular wafers held in compression by a biasing spring. The wafers include grooves molded into faces thereof forming passages between an outer portion and an inner portion. In a stacked configuration the wafers form a cylindrical filter element constituted by a radial labyrinth of passages which trap particles from fluid that flows between an outer surface formed by a plurality of outer portions of the stacked wafers and an inner surface formed by a plurality of inner portions of the stacked wafers.
One such system, as disclosed in U.S. Pat. No. 5,296,134 ("the '134 patent"), includes wafers stacked on a vertical guide that consists of three equiangularly disposed ribs. The wafers have their inner portions configured as a circular geometry dimensioned to fit snugly onto the vertical guide ribs forming an interior flow path in the interior of the column of stacked wafers. The circular inner portion of the wafers, although dimensioned for a snug fit, is free to rotate about the vertical guide ribs. Such a configuration, disadvantageously, permits mis-alignment of the wafers in the stack. This disadvantageous mis-alignment creates exposed surfaces that can become worn or broken, deteriorating the wafers and reducing the efficiency of the filtration mechanism.
The guide ribs of the '134 patent effect a flow path at the interior of the stacked wafers for filtrate, i.e. the fluid that has passed from the outer surface through to the inner surface having been filtered as it passed therethrough. The interior flow path not only acts as a conduit for filtrate to flow to a clean chamber, i.e. a repository for clean/filtered fluid, the interior flow path also acts as a backwash fluid flow path for fluid used to backwash the filter. When subject to a pressure surge, for instance resulting from clogged outer surfaces of the filter stack or from preferential flow of backwash fluid, extreme radial forces on the guide ribs can contort or permanently distort the vertical guide. Even minor contortions due to pressure surges acting on the vertical guide support ribs will result in filter element separation and inefficient filtration.
Prior art filtration systems, such as disclosed and claimed in the '134 patent, rely on the surface area of the wafers to trap particles and effect filtration. Therefore, the greater the surface area of the wafer, and overall filter element, the greater the efficiency of the filter. Throughput, i.e. the amount of filtrate processed, is also a function of the filtration efficiency or surface area. The stacked circular wafers with angularly disposed passages in systems such as in the '134 patent offer limited opportunities to increase surface area. Molding tolerances preclude significant increases in the density of passages, i.e. by reducing spacing therebetween. The possibilities for substantially increasing surface area which could result in substantial increases in throughput and/or reductions in filter size are very limited in prior art filter configurations.
Furthermore, prior art filtration systems such as in the '134 patent are mechanically arranged as a stack of wafers disposed between an upper and lower assembly plate. The lower assembly plate, while stabilizing the stack and somewhat overcoming the deficiencies of the fairly flimsy vertical guide rod, presents an impediment to the removal of filter cakes and particles during backwash. The lower assembly plate in prior art filtration systems is disposed directly in the path of backwash flow restricting such flow and preventing efficient cleaning of the filter.